BLOCK PRINTING, &C. JQ] 



tlian others, and on that account yielding more to the pressure 



of the springy leather; and the differences of hardness seemed 



to me to have arisen from the nature of the crystallization in 



cooling. The slower the cooling the larger will be the crystals Cause of this 



and the coarser the grain upon fracture. I did not pursue ^^^j^g^j^^^JJ^^'^^'* 



this object far; but I found, and I believe it is genera] to all crystallizatien 



castings, that the colder the metal, when poured out, the dens- *" ^^^"^ '"^' 



er specifically and more uniform in its fracture was the cast. 



These facts and observations seem to give probability to the 



following inference : that if two metals were cast by successive 



pouring out of the same pot, the latter would have more densi- I* 's thought 



. that a cast of 



ty and uniformity of aggregation and hardness, and would take metaj nearly 



a better figure upon the polisher, which though of pitch, and cold would take 

 if 1 1 .• •. u •. • .iM 1 1 .the best figure, 



supposed to have no elasticity, has, it is most likely, enough of 



spring to affect a figure which would be essentially injured by 

 a deviation of one millionth part of an inch. * 



3. I do not see how chromatic abberration is to be corrected On achromatic 

 by refraction towards the same parts, unless by mediums, such ^^^"P' 

 as described by Doctor Blair in his paper on aplanatic refrac- 

 tion, in the second volume of the Edinburgh Transactions, of 

 which an abridgment is given in our Quarto Series, vol. i. p. 

 1 . But these do not apply to the case mentioned by Mr. 

 Brewster. 



III. 



On the means of ohtaining Stamps (Cliches) with Moulds of 

 Plaster of Paris, Sulphur, or Sealingwax. By M. Darcet. 

 Read at the Society of Encouragement, the I2th of February, 

 1806.t 



Jl? OUNDERS use the term dicker for making impressions Art of stamp- 

 on metals without casting them in a mould; and that ofi^^'^.^gjj;*g'^^^ 



metals. 

 * Without recurring to optical principles, as to the figure of a 



speculum, it may be observed that a single stroke on the polisher 

 will sensibly alter that figure ; but that a great number of strokes 

 would be required to work out a scratch of one hundred thousand 

 to the inch in breadth. 



t Translated from La Revue Philosophique, Litteraire, et Poli- 

 tique, No. 10, for April, 1806, p. 1. 



cliche 



